1. Field of the Invention
The invention pertains generally to optical systems, e.g., optical fiber communication systems and optical mass storage devices, and more particularly to optical systems having antireciprocal polarization rotators.
2. Art Background
Optical systems for communicating and storing information are known and are now commercially significant. For example, an optical communication system, as schematically depicted in FIG. 1, typically includes a semiconductor laser which emits a light signal, e.g., an information-carrying light signal, to an optical fiber, which transmits the light signal to a photodetector. An optical mass storage device, as schematically depicted in FIG. 2, typically includes an optical disk which is capable of being, or has been, processed to store information. This information is encoded onto the disk (through processing) as regions of different optical properties, e.g., different optical reflectivity. The disk is read, i.e., the information stored on the disk is detected, by shining light from a light source, e.g., a semiconductor laser (typically through a beam splitter), onto the disk. The light reflected from the disk is then directed (i.e., reflected by the beam splitter) to a photodetector. Alternatively, the light transmitted by the disk is directed to a photodetector.
In a wide variety of optical systems, devices that rotate the polarization of linearly polarized light in the same sense irrespective of traversal direction are advantageously included. For example, the frequency and power intensity spectra of the light emitted by the semiconductor lasers employed in optical systems are altered when reflected light impinges upon the lasers. Such alterations are undesirable because they lead to errors in the detected information. Thus, efforts have been made to develop devices, called optical isolators, for isolating the semiconductor lasers from reflected light. An optical isolator based on rotation of linearly polarized light is exemplified, as depicted in FIG. 3, by a bulk magnetic garnet material, e.g., bulk single crystal yttrium iron garnet (Y.sub.3 Fe.sub.5 O.sub.12, called YIG) material, positioned between a polarizer and an analyzer. This optical isolator has been proposed for use with optical fiber communication systems operating at a wavelength of about 1.3 .mu.m because single crystal YIG is substantially transparent (at least 50 percent of the incident light is transmitted) at infrared wavelengths (wavelengths ranging from about 0.8 .mu.m to about 6 .mu.m). In operation, a magnet is employed to magnetize the YIG (in the direction of light propagation). Linearly polarized light emitted by a laser and transmitted by the polarizer is directed into the YIG material. Under the influence of the net magnetic moment within the (magnetized) material, the linearly polarized light experiences circular birefringence. (In a bulk material, e.g., bulk single crystal YIG, linearly polarized light may be represented as consisting of right- and left-circularly polarized components. Circular birefringence means the two components see different indices of refraction, resulting in one of these components propagating through the material at a faster speed than the other.) As a consequence, the light remains linearly polarized, but the polarization direction is continuously rotated in either the clockwise or counterclockwise (as viewed in FIG. 3) direction as the light traverses the material. (This phenomenon, commonly referred to as the Faraday Effect or magneto-optical rotation, is described in, for example, the McGraw Hill Encyclopedia on Science and Technology, 5th edition, Vol. 5 (McGraw Hill, 1982), page 314.) If the material is of appropriate dimension, the light is rotated through, for example, 45 degrees and is thus transmitted by an appropriately oriented analyzer. Reflected light transmitted by the analyzer also enters the YIG material and also undergoes a rotation of 45 degrees in the same direction as the light which originally traversed the material. Consequently, reflected light, after traversing the YIG material, is oriented at 90 degrees to the polarizer, and is thus precluded from impinging upon the laser. (The phenomenon by which a magnetized material rotates both forward and backward propagating linearly polarized light by 45 degrees (or an odd multiple of 45 degrees) in the same direction is denoted antireciprocal magneto-optical rotation. Devices which include such materials are referred to as antireciprocal devices.)
A second type of device based on rotation of linearly polarized light is a circulator. Such a device as employed, for example, in an optical communication system efficiently couples light signals from a semiconductor laser into one end of an optical fiber, and allows detection of counterpropagating light signals emanating from the same fiber end. One type of optical circulator (having a configuration suitable for efficiently coupling light into and out of an optical fiber end) is depicted in FIG. 4. This circulator, like the exemplary isolator, includes bulk single crystal YIG, and also includes a polarization sensitive reflector. In operation, a magnet is used to magnetize the YIG in the direction of light propagation. Linearly polarized light, e.g., horizontally (as viewed in FIG. 4) linearly polarized light, emanating from the optical fiber end, is directed into the magnetized YIG. (The optical fiber is, for example, a polarization preserving fiber. Alternatively, an appropriately oriented polarizer is positioned between a non-polarization-preserving fiber and the YIG.) If the YIG is of appropriate dimension, the light is rotated through, for example, 45 degrees (in the clockwise direction, as viewed from the fiber in FIG. 4) and is transmitted by the polarization sensitive reflector to a detector. Linearly polarized light emitted by a laser and oriented at, for example, -45 degrees (relative to the linearly polarized light emanating from the fiber) is reflected by the polarization sensitive reflector into the magnetized YIG. After propagating through the YIG, this light has been rotated 45 degrees (in the clockwise direction, as viewed from the fiber in FIG. 4), and thus enters the fiber horizontally linearly polarized.
While antireciprocal, light rotating devices based on bulk materials, e.g., single crystal YIG isolators and circulators, are useful, they are bulky (have typical dimensions of 3 mm by 3 mm by 3 mm), require the application of large magnetic fields (typically larger than about 1000 oersteds (Oe), are expensive (typically costing about 1000 dollars), and are thus not entirely commercially attractive. By contrast, a thin (having a thickness less than about 10 times the wavelength of the incident light) film waveguide antireciprocal device, e.g., an optical isolator or circulator, using planar magnetization would be a much more attractive device. For example, a thin film device would permit the use of guided wave optics (and thus eliminate the need for focusing lenses, not shown in FIGS. 1 and 2), require the application of relatively small magnetic fields (smaller than about 100 Oe), and be relatively inexpensive. In addition, it could also serve as a building block for integrated optical devices (an optical device which includes two or more components, performing different functions, and formed on the same substrate) useful in optical systems.
While thin film antireciprocal devices appear to be attractive, thin film waveguides are subject to linear birefringence. (In a thin film, linearly polarized light may be represented as consisting of two orthogonal, linearly polarized components. In one of these components, the electric field of the light (an electromagnetic wave) is oriented parallel to the film surface and is denoted the TM component. In the other component, the electric field is oriented perpendicularly to the film surface and is denoted the TE component. Linear birefringence means the two components see different indices of refraction, resulting in one of these components propagating through the film at a faster speed than the other. Regarding linear birefringence in thin film waveguides see, e.g., P. K. Tien, Applied Optics, Vol. 10, page 2395 (1971).) Thus, when traversing a magnetized thin film, light is subjected to elliptic birefringence, i.e., a birefringence which includes both a linear component and a circular component. As a consequence, initially linearly polarized light undergoes oscillatory rotation. (The distance traversed by the light in completing one oscillation is called the birefringent period, P.) This oscillation is depicted in FIG. 5 where the incident light impinges upon a magnetized thin film at an angle of, for example, 0 degrees (to the y-axis). While propagating through the film, the light is initially rotated through a relatively small angle, e.g., 3 degrees, in, for example, the clockwise direction. Further propagation produces a counterrotation to -3 degrees, and still further propagation to a distance P results in the light returning to its initial orientation (i.e., parallel to the y-axis). During this oscillatory rotation, the polarization of the light also varies continuously from linear to elliptic to linear. Because the amplitude of the oscillation is constant and, for most materials, small, e.g., 3 or 4 degrees, little or no net rotation is achieved. As previously discussed, an antireciprocal device must achieve a rotation substantially beyond that normally achieved in linearly birefringent materials, and on exiting, the light should be substantially linearly polarized to avoid, for example, optical power loss at the analyzer of an optical isolator. Thus, linearly birefringent devices have effects which, without compensation, preclude their advantageous use.
A magnetized, thin film optical device advantageously used as an optical switch or modulator, which compensates for the effects of linear birefringence, has been reported. (See P. K. Tien et al, "Switching And Modulation of Light in Magneto-Optic Waveguide Garnet Films", Applied Physics Letters, Vol. 21, No. 8, (Oct. 15, 1972), pp. 394-396, and U.S. Pat. No. 3,764,195 issued to Blank et al on Oct. 9, 1973.) This device, pictured in FIG. 6, includes a magnetic garnet film epitaxially grown on a garnet substrate, and a serpentine microcircuit formed on the upper surface of the garnet film. The microcircuit is formed so that the direction of current flow through the circuit is reversed every half birefringent period. Thus, the direction of magnetization (along the direction of light propagation) in the thin film is reversed every half birefringent period, which allows rotation beyond that normally achieved in a linearly birefringent material, but does not eliminate ellipticity in polarization.
A device which is subject to linear birefringence and is useful as a circulator or isolator has also been reported. (See R. H. Stolen et al, "Faraday Rotation in Highly Birefringent Optical Fibers," Applied Optics, Vol. 19, No. 6 (Mar. 15, 1980), pp. 842-845 and E. H. Turner et al, "Fiber Faraday Circulator or Isolator," Optics Letters, Vol. 6, No. 7 (July 1981), pp. 322-323.) This device includes a linearly birefringent optical fiber and a plurality of spaced magnets which magnetize (in the direction of light propagation) a number of fiber regions. The polarity of each magnet is the reverse of the previous magnet, while the spacing between the magnets is half the birefringent period of the fiber. The number of spaced magnets, and thus the number of correspondingly spaced magnetized fiber regions, is empirically chosen (in relation to the particular fiber) so that light exiting the last magnetized fiber region is elliptically polarized and the light intensities along the two birefringence axes of the fiber are equal. In operation, the elliptic polarization is changed to linear polarization by passing the light through a relatively long (about six birefringent periods), nonmagnetized and heated portion of the fiber extending from beyond the last magnetized fiber region. The amount of heat is determined empirically. Alternatively, the light is passed through an external compensator, which also converts the elliptic polarization to linear polarization. The settings of the compensator (needed to achieve linear polarization) are also determined empirically. Rotation beyond that normally achieved in a linearly birefringent material, elimination of ellipticity in polarization, and thus use as an antireciprocal device are achieved. However, those engaged in the development of optical systems have sought, thus far without success, linearly birefringent optical isolator/circulator devices in which elliptically polarized light is conveniently (rather than empirically) converted to linearly polarized light.